Yahoo Answers is shutting down on May 4th, 2021 (Eastern Time) and beginning April 20th, 2021 (Eastern Time) the Yahoo Answers website will be in read-only mode. There will be no changes to other Yahoo properties or services, or your Yahoo account. You can find more information about the Yahoo Answers shutdown and how to download your data on this help page.

Trending News

Promoted
josh l
Lv 5
josh l asked in Cars & TransportationMotorcycles · 9 years ago

rich running bike. if u know mikuni carbs answer this?

we've come the the conclusion my buddy's engine is running rich,i told him we need to just drop the needle a bit or buy a smaller main jet(fuel). he believes we can jus leave da carb alone an pull da filter out the airbox an get more air goin in w/ that gas, idc what he does,if he wants 2 run it w/o a filter an ruin his engine so be it,he's scared messin w/ the carbs adjustments will mess something up terribly, so yall get on here and suggest and im gonna show it to him what the majority of yall think. post w/ what u think we need to do.remember its runnin rich(2 much fuel and/or not enuf air) so would you go w/ a smaller main jet(fuel), a bigger main air jet(to allow more air in),drop the needle, or maybe go ahead an do a lil of both types of jets, i dont believe the pilot jet is a problem here but the pilot air jet could also need tweaking if we dont mess w/ the main air jet,so wud u go w/ less fuel or turn a air screw out an allow more air in or both?.... id hate 2 see him mess up a good runnin but slo engine cause he did away w/ his air filter.

Update:

i buy terrible runnin bikes that kill w/ the chop of a throttle for a living an get them runnin right an sell for a profit.as long as its not in the bottom end of the motor i can fix it. he's a bit scared tho to mess w/ sumthin an doesnt realize what goin w/o an air filter can do

Update 2:

we dont got no paved roads aroun here for about 5 miles.an he ran it w/o the filter an it still boggs.he got the bike like this and didn pay much for it.were jus tryin lean the mixture out as i stated before.i guess we'll try smaller main jet first

Update 3:

woops my bad i meant five miles,didn mean 2 use #'s again.damn it i dont it again

Update 4:

cause i know this, the whole bottom of the plug is sooty black. the tip,the porcelain part,the base and up to about 4 threads on the spark plag.its choppin out at about more than 1/2 throttle

3 Answers

Relevance
  • bikinkawboy
    Lv 7
    9 years ago
    Favorite Answer

    First, try actually spelling words and don't use "da" and "2". That texting business may be ok for you young kids but if you want advice from an experienced old fart like me, spell it correctly.

    Your buddy is on the right track partially. Remove the filter and see how it runs, if better then you know the filter needs to be replaced. Don't go running it down some dusty dirt road, but on a paved road you'll be ok. Back in the 50s and 60s some European bikes didn't have air filters when the came out of the factory.

    If the filter seems ok, run it wide open throttle under load. If it runs rough and blubbers, or runs roughly but with soft jerks, you know the main jets are too large. If it misfires with hard, sharp jerks and runs better when you close the throttle slightly, you know the main jets are too small. Mikuni jets are sized in 2.5 increments, i.e. 105-107.5-110 . The numbers identify the amount of fuel the jet will flow with a certain fuel pressure for a given time period.

    Once you have the main jet sized correctly, you can then go to adjusting the needle up or down if needed. Starting in 1980, the needles that came installed on the bike had one clip position only, thanks to EPA. I'm not sure if or when they went to multi position needles again. If you need to richen up a 1980+ Mikuni, you can take a soda pop can and make tiny washer like spacers to place under the needle clip, raising it. Lowering them out isn't really possible for the ordinary Joe.

    If the engine stumbles or falters when going from the closed throttle position to a small throttle opening, the pilot jet screws need to be adjusted (or the pilot jets are plugged up from setting around with stale gas). Most Mikuni jet screws control fuel flow and turning counterclockwise richens the mixture. However, some control air flow like those on my '79 Kaw. Turning the screw counterclockwise leans out the mixture.

    Summed up, you need to first make sure the air filter isn't plugged, then check and adjust main jet size if needed. Then adjust the jet needle and lastly, the pilot jet screw. Incidently, if you or someone else soaked the carb bodies in professional carb cleaning fluid for any length of time, that stuff will erode brass. That will enlarge the hole in the needle jet, the brass jet the jet needle fits into. There's a big difference between the jet needle and the needle jet. If you need more info, email me.

    Source(s): Now see why you should placate old farts like me? Because unlike you young sprouts, we know what we are talking about.
  • Anonymous
    9 years ago

    Take to a qualified mechanic.

    When did it start running rich? This will help figure out why.

    You should not have to change anything in the carb if it ran correct before.

    Find out the problem. Don't mask it.

    Good luck.

    Source(s): www.ridefree.com
  • mn
    Lv 7
    9 years ago

    WHAT BIKE??

    how do you know its running rich???

    the conclusion you came to maybe wrong...

Still have questions? Get your answers by asking now.